BSPD

BSPD RZ-XX6-C/E — Brake System Plausibility Device

Ashwa Racing 27
Designed by Aditeya Sarkar

What is a BSPD?

The Brake System Plausibility Device is a mandatory safety circuit in FSAE/Formula Student vehicles. Its job is to cut power to the motor (via the Shutdown Circuit / SDC) if the driver simultaneously applies heavy braking and demands significant throttle — a condition that should never happen during normal operation and indicates either a pedal fault or a stuck throttle.

This design targets the FSG CV+EV rules (with CV mode support via a jumper) and is built entirely out of discrete analog logic — no microcontroller involved.

Block Diagram / Signal Flow

Connectors (BPS, TPS, SDC)
        │
        ▼
Current Sensor (EV only)          ←── Biasing circuit converts ±4V diff → 0.5–4.5V
        │
        ▼
Implausibility/Plausibility Check  ←── 500ms timer + 10s timer + comparators
        │
        ▼
SCS Check (Short Circuit Safety)
        │
        ▼
Reset System (SR latch + Power-on reset)
        │
        ▼
Relay (SDC_OUT → cuts motor power / fuel pump)
        │
        ▼
Back to Connectors

Signal flow as annotated in the schematic (Sheet 1/9)

Schematic Overview

The top-level sheet ties everything together. The core logic sits around a SN74HCS14DRQ1 (Schmitt-trigger inverter, U1) and a SN74HCS32DRQ1 (OR gate, U2), with the relay controlled by a NMOS gate driven off RELAY_NMOS_G_BAR.

Subsystems

1. Power Supply

Input is +7V from the car's GLV (Grounded Low Voltage) supply. The board generates a regulated +5V rail using an LDL1117S50R LDO. Protection is handled by:

• F1 — Polyfuse for overcurrent
• Q2 (STR2P3LLH6) — P-channel MOSFET for reverse polarity protection, driven by a BC856BS transistor pair (Q1A/Q1B)
• D1 (NZH16C) — Zener clamp

For EV operation, isolated ±12V rails are generated via an MPA1212S DC-DC isolator (PS1), referenced to ISO-GND, with a Y-capacitor (C3, 10µF) for common-mode filtering.

Total board power budget: ~145–150mA (LEDs 40mA + ICs 40mA + dividers ~5mA + relay 40mA + misc).

2. Implausibility & Plausibility Check

This is the heart of the BSPD logic. Three comparator circuits run in parallel:

Implausibility Check (bottom-left, Sheet 9)

• Uses TC33X-2-103E references and LM339LV-Q1 comparators
• Both BPS (brake pressure) and TPS (throttle position) must exceed their respective thresholds simultaneously to flag implausibility
• Output: Implausibility signal (HIGH = fault)

500ms Implausibility Timer (top-left, Sheet 9)

• Uses a DMG2305UX-7 complementary MOSFET pair to charge/discharge a capacitor (C12, 10µF + C13) at a controlled rate
• When implausibility is sustained for >500ms, 500ms comparator output goes HIGH
• The DWC2305UX-7 N-channel requirements: Vth ~1V, Ids up to 10mA → 2N7002K used
• The P-channel: Vth ~−1V, Ids ~10mA → same DWC2305UX-7 (check datasheet for exact variant)

10s Plausibility Timer (top-right, Sheet 9)

• Similar RC-based timing circuit; capacitor (C13) only charges when implausibility is active
• Output: 10s comparator output — used to latch the fault via the SR latch if the 10s window is exceeded
• Reset_Signal_Low can discharge the cap and reset state

3. Memory Element (SR Latch) + Power Reset

SR Latch (Sheet 4, left)
Built from SN74HC02-Q1 NOR gates (U4A/U4B):

Q drives Q_Output and Q_OUT_BAR. The relay state is: Relaybar = NOR(SCS_bar, Q).

So the SDC relay opens (cuts power) only when Q = 0 and SCS_bar is LOW — i.e., no fault and no short circuit detected.

Power-on Reset (Sheet 4, right)
Uses an LM339LV-Q1 comparator (U5A) with R16 (100k), R17 (47k), and C7 (10µF) to generate a ~200ms HIGH pulse on PWR_RST at power-on. This forces the latch into a known state (R = 1 momentarily → Q = 0). Once past the PoR window, R → 0 and normal latch operation resumes.

Two push-buttons (SW1, SW2) provide manual Reset_Signal_High / Reset_Signal_Low for bench testing.

4. SCS Short Circuit Check

The SCS (Short Circuit Safety) check uses LM339LV-Q1 comparators (U5B/U5D/U5C) to detect wiring faults — specifically shorts to GND (<0.05V) or shorts to +5V (>4.95V) on the BPS and TPS lines.

• Normally: SCS_Check = HIGH (no fault)
• On short detection: goes LOW, which blocks the relay from closing regardless of Q

Thresholds set by 100k/1k voltage divider pairs on each input.

5. Current Sensor (HB0 300) Biasing (EV Only)

The current sensor (external, connected via J3) outputs a differential signal in the ±4V range. Sheet 6 handles the level shift to a 0.5V–4.5V single-ended signal suitable for the comparators.

• BAT42W Schottky diodes (D3–D6) clamp Vs+ and Vs− to the 0–5V range
• TLV9002-Q1 op-amps (U6A/U6B) in a difference amplifier config with a +2.5V mid-rail reference
• Output: CS_OUT → fed into the implausibility comparator as the throttle-side signal in EV mode
• A jumper on Sheet 8 allows bypassing this for CV (combustion) mode, where TPS1_IN connects directly

6. Relay

The SDC relay is an OJE-SH-105LMH (U7), driven by an NMOS switch (MMFTN3422K, Q3) on the low side. SDC_OUT drives the relay coil. A S3B5MB diode (D7) provides flyback protection. A blue LED gives constant visual indication of relay state.

7. Connectors

All external signals come through TE Connectivity 1744426-series connectors:

BPS scaling: R42 (200Ω) + R43 (430Ω) divider sets BPS_SCLD in the 0.88V–4.4V window.
ESD protection on BPS line via ESDCAN01-2BLY (D9).

8. Probing Points

16 dedicated test points for bench validation:

Key ICs

KiCad Version Compatibility

This schematic was designed in KiCad EDA 10.0.0.

If you're on KiCad v9 and need to open these files, a downconverted v9-compatible version is available — reach out or raise an issue. The downconversion removes v10-specific fields (like uuid blocks and certain footprint attributes) that cause parse errors on older installs.

Repo Structure

BSPD/
├── BSPD.kicad_sch              # Top-level schematic
├── Power Supply GLV.kicad_sch
├── Probing points.kicad_sch
├── Reset system.kicad_sch
├── SCS check.kicad_sch
├── Current Sensor.kicad_sch
├── Relay.kicad_sch
├── Connectors.kicad_sch
├── IMP_P_Check.kicad_sch       # Implausibility/Plausibility check
└── README.md

Team

Ashwa Racing — 27
R.V. College of Engineering, Bangalore

Electrical & Testing Subsystem
Schematic design: Aditeya Sarkar